Cutting inserts with one or more layers of coating, for example, thin film physical vapor deposition (PVD), chemical vapor deposition (CVD) and thick hard coatings in the form of single or multilayer architectures has become standard practice for enhancing wear and corrosion resistance in the cutting tool industry. In production, coating quality is related to various aspects, such as: thickness, adhesion to the substrate, hardness, surface roughness, visual appearance, composition, microstructure, residual stresses, and the like. Depending on their intended application, hard coatings usually range from a few hundred nanometers up to about 50 micrometers in thickness. To consistently produce a coating of a certain thickness, a good control over the deposition rate and other process parameters is required. In the practice, process optimization is often aided by trial and error, or DOE's study of many parameters on a large sample set, thus making it necessary to perform coating thickness measurements on a regular basis.
This importance of coating thickness assurance has been widely reported in the literature. Besides being a key quality aspect in coated parts, thickness also influences the physical, thermal, mechanical, corrosion and tribological response of the coating/substrate system. In multilayer coatings, these properties have been related to the coating architecture, i.e., number, distribution, proportion, composition and thickness of the individual sublayers. Moreover, the residual stress state of the coating and its adhesion to the substrate are known to be thickness-dependent parameters. Thus, thickness continuity is a key aspect for quality control for a cutting insert having a single layer or a multilayer architecture.
Currently, there are three methods that are widely employed for measuring thickness of single and multilayer coating architectures, namely: Calotte Grinding Method (CGM), Glow Discharge Optical Emission Spectroscopy (GDOES), X-ray fluorescence and Metallographic Micro Polishing (MMP) followed by optical or scanning electron microscopy (SEM) analyses.
GDM is a micro-abrasive procedure in which coating thickness determination relies on the measurement of the circular projections of a calotte-shaped wear scar produced by a hard steel ball rotating freely against the specimen until the coating has been perforated. A micro-abrasive diamond suspension is delivered near the contact to ensure even surface wearing and well defined edges around the calotte.
GDOES is an advanced spectroscopic technique that allows the determination of chemical composition profiles as a function of depth from a few hundred nanometers up to about 50 micrometers. In this technique, the coated sample (cathode) is placed in a copper electrode (anode) and a discharge is set between them, thus producing erosion (sputtering) of the sample surface at a controlled rate. The released atoms are excited in Argon plasma and photons are emitted as they return to their fundamental energy level. These photons are subsequently collected and sent to an optical spectrometer consisting of an array of photomultipliers that quantify elemental concentrations as a function of the intensities of the photonic emissions. Therefore, the thickness of a coating may be estimated as the profile depth at which marked transitions occur between higher to lower atomic concentrations of the coating elements and lower to higher concentrations of the substrate elements.
MMP relies on the metallographic preparation of the coated sample by cross-section polishing, followed by coating thickness measurements using commercial microscopy techniques. The MMP specimens are typically hot mounted in an epoxy resin containing mineral and glass fillers that provide optimum planeness and excellent edge retention during the cross-sectional preparations. The grinding and polishing steps are conducted using a semi-automatic specimen mover installed on a disc-type metallographic machine, achieving a maximum sliding speed of around 30 mm/s. Plane grinding is performed using resin-bonded diamond abrasive disc, lubricated with water under a constant load, and fine grinding is done using grinding paper in several steps.
Due to their inherent differences, each method provides results at various levels of cost, accuracy and time. For instance, the high precision of MMP is the main advantage of this method. However, sample preparation is time consuming, which typically takes about forty minutes to prepare the sample and another fifteen minutes to measure the coating thickness. GDOES provides quick results, but the equipment and testing costs are high. The CGM is the quicker and less expensive.
Due to the amount of time necessary for each of these techniques, the current techniques only support the research and development phase of creating the coating architecture, rather than the manufacturing phase in which a sample lot of cutting inserts can be continuously inspected for quality control on a regular basis. In addition, if an inconsistency in the thickness of the coating is identified in a manufacturing lot, then the lot needs to be quarantined and inspected. Thus, there is a need to provide a method for measuring the thickness of one or more layers of coating of a cutting insert that can be performed in a timely, cost effective and non-destructive manner.